Fungi

Fungi are everywhere - beneath your feet, almost everywhere you look, and even in the air you breathe. Without these strange and fascinating life forms, neither we, nor the inhabitants of our native forests, would survive for long.

Walk through the forest, and you are surrounded. The mushrooms and toadstools with which we are familiar are just a tiny glimpse of the body of a fungus. Fungi are everywhere - beneath your feet, on virtually every surface you rest your eyes upon, even in the air you breathe. Far from this being a disturbing prospect, rest assured that without these strange and fascinating life forms, neither we, nor the inhabitants of our native forests, would survive for long. However, the mushrooms and toadstools with which we are familiar are just a tiny, partial glimpse of the true bodies of fungi.

Fungal physiology

Scientists classify living things into a number of different kingdoms, with plants and animals being just two of them. There was a time when fungi were thought of as plants but they have since been awarded a well-deserved kingdom of their own. In fact, fungi are more closely related to animals.

Fungi are made up in quite a unique way and don't have cells like those of a plant. Instead (with the exception of yeasts), their smallest units are tiny threads known as hyphae. Many of these can only be seen with a microscope. Collectively these threads form a network called a mycelium. A mycelium can be miniscule: spreading though the body of a dead fly; or it can rank among the largest, heaviest and oldest living things on the planet. One example is the gargantuan Armillarea ostoyae that occupies 965 hectares (2,384 acres) in Oregon's Blue Mountains in the United States. It is thought that this ancient organism could be between 2,400 and 8,650 years old!

Astonishingly, while hyphae can be tiny, there can be 100 metres of them in a gram of soil, and in a hectare (2.5 acres) of British woodland, there may be well over three and a half tonnes of fungi! If you pick up a handful of leaf litter in the forest, you are likely to expose the slightly furry looking network of fungal mycelia. While the individual threads are microscopic, there are so many of them, often clustered together, they can become visible to the naked eye.

The cell walls of fungi are made out of a tough substance called chitin (pronounced 'kytin'). While chitin is similar to cellulose, which helps form the cell walls of plants, it is a different substance and is actually the same material that makes up the hard external skeletons of insects.

Feeding habits

Whereas plants get their energy directly from the sun and atmosphere using photosynthesis, fungi get theirs by digesting living or dead organic matter, as animals do. Fungi obviously have no mouths or stomachs and instead they work their way through or over their food, absorbing nutrients directly through their cell walls. Nutrients with simple molecules, such as sugars, can be absorbed fairly readily. Larger, more complex molecules, such as proteins, are harder to tackle, and the fungi must then make use of various enzymes (chemicals that help to dissolve and simplify the molecules) so that they are easier to absorb.

Reproduction

Sex is a complex business for fungi. Many fungi can reproduce sexually and asexually. Under certain conditions they can send up a fruiting body without interacting with another fungus. This is asexual reproduction and has the advantage that it can happen quickly, to make the best of a small window of good conditions. The fruiting body will release millions of its microscopic spores, a tiny proportion of which will germinate.

Sexual reproduction enables the next generation to benefit from the genetic material of both parents, allowing it to develop new adaptations. Hyphae from two different fungi of the same species intertwine and then send up a fruiting body or mushroom, again releasing spores, but containing genes from both parents. The fruiting body is actually made up of a collection of the same hyphae that form the mycelium, just more densely-packed. Mushrooms have a seemingly miraculous ability to appear over night as they use hydraulics to 'inflate' the hyphae with fluid so that they grow rapidly, and can push their way up through some surprisingly hard surfaces. Inkcaps (Coprinus spp.) for example can even push their way up through tarmac!

Whether sexual or asexual, the result is that innumerable spores are released into the air. Some species can release tens of millions of spores in a single hour - so it's perhaps just as well that only a small proportion of them actually germinate! Spores from the mushroom are then carried on the breeze, often many miles from their source.

Fungi come in many strange - and often very beautiful - shapes and sizes, but are placed in two main classes (along with a third class, which contains certain moulds), based on the structures of their fruiting bodies. Some are 'spore-shooters', otherwise known as the ascomycetes. The spores of these are produced in flask-shaped sacs, called asci. Pressure builds up in these sacs and the spores are actually shot out when ripe. The ascomycetes include yeasts, as well as more obvious denizens of our native pinewoods such as the strangely-shaped false morel (Gyromitra esculenta). The other, more common class are the 'spore-droppers', more formally known as basidiomycetes. These have either gills or pores on the underside, with spores being produced on tiny club-shaped cells, from which they are released to travel on the wind. These include all the familiar forms - mushrooms and toadstools, as well as bracket fungi, all of which can protect their spores from the elements with their waterproof caps.

Other kinds of dispersal are also found: puffballs (such as Lycoperdon spp.), eject a puff of spores when a drop of rain hits the surface of their fruiting bodies, while the stinkhorn (Phallus impudicus) smells of rotting flesh, attracting flies which unwittingly disperse the spores on their feet.

Partnerships

Especially intriguing are the mycorrhizal fungi. Mycorrhizal relationships are fascinating partnerships that take place when the hyphae of certain fungi wrap around, or penetrate the roots of a plant, whereupon a mutually beneficial exchange takes place. The fungus, which cannot obtain energy directly from the sun itself (as it lacks the chlorophyll found in plants), is able to obtain sugars that the plant produces using photosynthesis. In return the fungus provides the plant with vital nutrients that it extracts and transports from the soil, and that would otherwise be unavailable to the plant. Surprisingly, most of the plants in virtually all of the world's terrestrial ecosystems rely on these relationships for their healthy growth. It gives some perspective on the importance of fungi when we consider that without them the world's forest ecosystems would collapse. Among the mycorrhizal fungi in native pinewoods are species such as the chanterelle (Cantharellus cibarius) and various Boletus species, as well as the familiar red-with-white-spots fly agaric (Amanita muscaria), that grows in mycorrhizal association with birch trees (Betula spp.).

Ants are known to live symbiotically with fungi, particularly in the tropics, with leafcutter ants (e.g. Atta spp.) farming fungi for their own consumption. Recent observations in Glen Affric suggest that wood ants (Formica lugubris) possibly have similar interactions with fungi, and further investigations are required to reveal the nature of this relationship.

Parasites

Fungi that feed on living things are parasites. Some parasitic fungi simply weaken their hosts, while others kill them. Examples include the aspen bracket fungus (Phellinus tremulae), and honey fungus (Armillaria mellea), with its thick, black, bootlace-like rhizomorphs, which are effectively giant hyphae. Many of these kinds of fungi dwell within their hosts for some time before attacking and killing the tree.

This in itself creates superb deadwood habitat for a host of other species, from beetles and flies (the larvae of many species feed on dead wood) to crested tits (Parus cristatus) and ospreys (Pandion haliaetus), which nest in dead trees. When a tree is killed, it also provides an opening for young trees and other plants to grow, thus enriching the structural diversity of the forest.

Recyclers

Without fungi, the forest would pile up with layer upon layer of needles, leaves and other dead matter. The fungi that feed on dead organic matter are called saprophytes. The key role of these forest recyclers is to break down dead matter and return the nutrients to the soil to become available to plants once again. Leaf litter, dead animals, dead wood - in fact, anything that dies in the forest will be colonised by fungi (along with other decomposers) and eventually reduced to soil.

The role of fungi in breaking down dead wood is especially crucial. Lignin is the substance that makes wood stiff, and it is so tough that animals cannot digest it. However, certain fungi are able to biodegrade this substance using particular enzymes, thus allowing the vast amounts of dead wood in a natural forest to be broken down.

A forest feast

The fruiting bodies of fungi provide an abundance of food for the wildlife of the forest. Squirrels (Sciurus vulgaris) store fungi in the tops of trees to eat through the winter. Voles (Microtus agrestis) and other rodents also gnaw on this welcome feast: their teeth marks, and the marks of the rasping mouthparts of slugs, can often be seen on the caps of mushrooms.

Fungi can be filled with life. As any fungal forager will tell you, if you pick a fungus that?s past its best, the chances are it will soon be riddled with maggots. These are the larvae of various fungal gnats and other insects. Bracket fungi such as the hoof fungus (Fomes fomentarius) are a haven for insects and can support a mini-ecosystem in their own right. A study of the hoof fungus in Swedish forests revealed 27 insect species that live within the brackets, including various fungivorous beetles and moths.

A forest full of fungi

Autumn is the best time for fungus lovers to walk through a native pinewood. Not because there are more fungi, but because many of the fungi that are there all year round become more conspicuous, sending forth the familiar mushrooms and toadstools. Since they depend on moist conditions to feed and grow, autumn is an ideal time for reproduction. The familiar smell associated with autumn woodlands is all down to fungi working their way through the soil.

Our native woodlands have an abundance of fungi. In the Caledonian pinewoods at Abernethy for example, 699 species have been recorded so far. Perhaps because they are so hard to see, fungi have been largely overlooked in spite of their importance. About 69,000 species of fungi have been discovered worldwide, but it is thought that as many as 1.6 million actually exist! So in spite of the fact that fungi surround us, there could be many more to discover, even on our own doorstep.

Dan Puplett

Sources & further reading

Allaby, M. (1986) The Woodland Trust Book of British Woodlands. David & Charles: Newton Abbot.

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About Trees for Life

The core purpose and vision of Trees for Life is to restore the Caledonian Forest and all its constituent species of flora and fauna to the Scottish Highlands. By engaging volunteers to achieve this, we aim to provide a powerful and educational experience that will promote the work of restoration to wider audiences. Through providing a demonstration of the techniques of ecological restoration, we aim to inspire similar projects elsewhere in the UK and around the world.